It is Tony Blair's boast that he wrote every word in longhand "on hundreds of notepads". That I believe. He was the most brilliant communicator of his era as a platform speaker or television interviewee, but he can be a ghastly writer. Anyone thinking about taking this journey needs to be given a travel advisory: much of the prose is execrable.

No cliche is avoided. Loins are girded, leashes are strained at, die are cast, lights appear at the end of tunnels and wounds are rubbed with salt. The Vatican is "an amazing place". Princess Anne "is a chip right off the old man's block". Princess Diana "captured the essence of an era". Derry Irvine is "like the proverbial dog with the bone". Many of the chapters are as badly planned as the invasion of Iraq. There are abrupt jumps from this year to that and back again. He will launch into one subject and then suddenly lurch off in an entirely different direction. The text is littered with the apology: "Anyway, I digress." Sentences begin with redundant clunkers: "Funnily enough…" or '"Needless to say…" One paragraph concludes: "Blimey, get a life." Another ends: "blah, blah, blah." There are passages that read like a Craig Brown parody in Private Eye. Of Condoleezza Rice, Blair writes: "She is also a classic example of the absurdity of people with experience and capacity at the highest level not having big political jobs after retirement from office. But that's another point!" There is a smattering of scenes with Cherie that will be strong contenders for the Bad Sex award. By page 544, he is inviting us to join him on the toilet. "I like to have time and comfort in the loo."

I could say that it is a pity that Tony Blair did not employ a ghostwriter to prettify the prose and organise his recollections more elegantly. I could observe that he is straining after the faux-intimate style of the autobiographies of footballers or models, and that was only to be expected from the politician who turned himself into Britain's first celebrity prime minister. Funnily enough, as he would write, the direness of much of the prose helps to give this autobiography some of its authenticity. The book has a kind of integrity. I say kind of because it is a rare political memoir – and this is not one of them – that is unflinchingly candid. He is slippery on inconvenient facts and passes over issues that are too painful to confront. He is not always reliable about either chronology or detail. Events and relationships are sanitised.

That said, this is a more honest political memoir than most and more open in many respects than I had anticipated. He is compellingly candid about how scared he was when he first became prime minister. He was masterful at masking his fear from the public, but it prevented him achieving as much with his first term as he might have done. Even after many seasoning years in office, his "demons" still urged him to "run away". I am sure many politicians harbour such feelings, but no former prime minister has ever been so open about it. Talking about his strengths, he describes himself as a "manipulator". He is unusually direct about his calculations, even when they don't reflect well on him, and his motivations, albeit in a pop-psychological way that frequently makes you wince. So long as the subject area is not Iraq, he is often brutal with himself about his mistakes. He admits to stretching the truth beyond "breaking point" to secure a settlement in Northern Ireland. Even when the lies are told in a noble cause, few politicians are honest enough to admit that they sometimes feel compelled to be deceivers.

The book adds little to what has already been written about his relationship with Gordon Brown. We had already gathered that he found his friend turned rival "strange" and "maddening" – and much worse that he does not say – but for complex reasons could never bring himself to deal with the chancellor who relentlessly sabotaged his premiership. The TB-GBs were fiercely denied when some of us started to expose their uncivil war long ago. It is useful nevertheless to have further corroboration supplied by Blair himself. One of his most important confessionals – and this reflects worse on him that it does on the other man – is that he knew that Brown would be a disastrous prime minister but did nothing serious to seek an alternative outcome.

This is not a bitchy memoir. That reflects the nature of a man who tends to look for and try to exploit the good in other people rather than the bad. That trait equipped him to woo both Gerry Adams and Ian Paisley for peace in Northern Ireland, which was a plus. On the negative side, he still can't see what is wrong with Silvio Berlusconi. There is no resiling from his admiration for George Bush, who is lauded as a man of "genuine integrity" and "a true idealist". He finally admits that he wanted Bush to win a second term, an extraordinary position for a Labour prime minister to have got to and the most vivid example of how Blair was bent out of shape by that alliance. He struggles to account for the terrible consequences when he hitched his liberal interventionism to Bush's crude neo-conservatism. The Iraq chapters are the least revealing. They are unlikely to change anyone's mind.

In his early years in office, he was often called a chameleon. He has tried to make sense of his career by defining himself as a leader who started out as a successful populist who was too eager to please and matured into a statesman prepared to be a hated "minority of one" for the causes that he believed in. Hence the title. I finished his autobiography still thinking of Tony Blair as a kaleidoscope. He can be charmingly self-deprecating one moment, and repellently vain the next. Banalities tumble across these pages, but there are also thoughtful and significant meditations about modern politics. This autobiography, like its author, has many faces: important and infuriating, trite and profound, cynical but also optimistic, world-weary and yet often quite naive, racked with anxieties about some things and evangelical in his certitudes about others, intellectually lazy and confused about many issues but more often than not utterly clear-sighted when it comes to the big ones. When he says the world is a place of contradictions, Labour's longest-serving prime minister is really talking about himself.